When implanting hip prostheses, trialing systems are used to verify that the surgeon is using the correct size of shell and head. The present invention provides an improved trialing system for use with constrained hip implants, including particularly bi-polar hip implants. Examples of bi-polar hip implants are found in U.S. Pat. No. 4,241,463 (Khovaylo) and U.S. Pat. No. 4,798,610 (Averill et al.), both of which are incorporated herein by reference.